Abstract

We present evidence for the occurrence of an insulating phase both at zero and high magnetic fields, observed in extremely dilute two-dimensional hole systems in GaAs-Al1-xGaxAs quantum wells. At the highest hole concentrations the insulating phase is consistent with the magnetically-induced Wigner solid. As the hole concentration is persistently reduced by illumination of the structures, an insulator-metal-insulator transition develops in the magneto-transport around filling factor nu = 1 and eventually the structure becomes insulating at all magnetic fields. The zero-field insulator emerges at a critical value of the dimensionless inter-particle separation, r(s), of around 14, the highest value yet reported for this transition. We discuss these observations in the light of recent studies of the Wigner solid and the Hall insulator and invoke a model of the recombination kinetics of our experiments which accounts for the reduction in hole concentration under illumination. (C) 1997 Published by Elsevier Science Ltd.